This invention relates to a monochromator for a beam of photons having a wide energy range. More particularly, this invention relates to a monochromator for continuous spectrum x-ray radiation, such as Bremsstrahlung radiation and especially synchrotron radiation.
In recent years, synchrotron radiation has been developed as a source of x-rays for experimental use. Synchrotron radiation offers many features which are of interest in x-ray experimentation. The radiation has a continuous wavelength spectrum which, depending on the maximum energy of the orbiting electrons, may well extend from 0.1.ANG. x-rays to visible light. Synchrotron radiation has a high flux, is highly collimated, and the radiation emitted in the orbital plane may be polarized up to 100%.
Various combinations of these features may be required for different types of x-ray experiments. For example, a beam of high flux and low energy resolution may be adequate for diffuse scattering measurements, while inelastic scattering experiments might require higher energy resolution. Absorption measurements such as those performed in extended X-ray absorption fine-structure measurements could have still further requirements. In addition, the availability of synchrotron radiation could make new experiments and applications of x-rays feasible for the first time. These new applications could impose still further requirements.
These different requirements must be met by the optical design of the synchrotron beam line. In particular, the monochromator has a major role in this context. Unlike conventional x-ray machines which used characteristic lines and, therefore present minimal problems of monochromatization, the continuous spectrum of synchrotron radiation or that of the Bremsstrahlung presents the problem of monochromatization if the experiment requires only one precisely known wavelength, or if the wavelength used must be known precisely as it is varied as a function of time. A monochromator for synchrotron radiation should be adaptable to a variety of conditions.
It is known that double monochromators, that is, monochromators having two diffracting crystals operating in tandem, offer improved resolution of x-ray beams. The theory of double monochromators is set forth by R. W. James, The Optical Principles of the Diffraction of X- Rays, 1965, Cornell University Press, Ithaca, New York, pp. 304-318, and by J. W. M. DuMond, Physical Review, 52, (1937), 872. Much of the effort in designing early double monochromators was based on this theory. In particular, focusing monochromators are reviewed by J. Witz, Acta Crystallographica, A25, (1969), 30.
More recently double monochromators have been applied to the unique characteristics of synchrotron x-rays. The optics of focusing and non-focusing monochromators for synchrotron radiation has been reviewed by J. B. Hastings, Journal of Applied Physics, 48, (1977) 1576. Multiple Bragg reflection monochromators for synchrotron radiation have been described by J. H. Beaumont and M. Hart in the Journal of Physics, E7, (1974), 823. In these monochromators each crystal is cut into a pair of reflectors to provide multiple reflection. Each pair of reflectors must be cut from one monolithic perfect crystal to ensure that their relative orientation is correct. This type of monochromator generally does not eliminate higher harmonics.
Several types of perfect crystal monochromators for use with synchrotron radiation have been reviewed by U. Bonse, G. Materlik, and W. Schroder in Journal of Applied Crystallography, 9, (1976), 223. These monochromators utilized the multiple reflection groove crystal alone and in combination with other perfect crystals. It is shown that higher harmonics can be eliminated if the monochromator system is dispersive, that is if two different types of crystals are used simultaneously.
In other monochromators the two crystals are mounted in a device known as a Thompson bearing. This device rotates the two crystals in tandem and also controls translational movement of the crystals with respect to one another. By rotating and translationally moving the crystals the monochromator can achieve a range of Bragg angles so that the radiation having the range of corresponding wavelengths can be monochromatized. The Bragg angles available with the Thompson bearing generally range from about 6.degree.-60.degree. to about 45.degree.-60.degree. which corresponds to a maximum energy of about 25 keV. The bearing is designed so that the translational and rotational movements of the crystals are all interdependent and operated by a single motor. The machinery of the device must be extremely accurate to maintain parallelism between the crystals. In addition, the crystals must be very carefully and precisely mounted in the bearing.
All the prior art monochromators discussed are advantageous for particular types of experiments, yet no one monochromator has sufficient flexibility to accommodate different experiments having widely disparate requirements. For example, a monochromator suitable for non-dispersive work could not provide a low dispersive beam. Similarly, a monochromator designed for low energy experiments could not provide high energy output. It would be desirable to have a single monochromator which would be adaptable to a variety of experimental conditions.